The National Audubon Society's Christmas Bird Count (CBC) is an early-winter bird census, where thousands of citizen scientists across the US, Canada and many countries in the Western Hemisphere, go out over a 24 hour period to count birds.

Count volunteers follow specified routes through a designated 24-kilometer diameter circle, counting every bird they see or hear all day. It’s not just a species tally—all birds are counted all day, giving an indication of the total number of birds in the circle that day. All CBC’s are conducted in the period from December 14 to January 5 each season, and each count is conducted in one calendar day.

The data collected by observers over the past century allow researchers, conservation biologists and other interested individuals to study the long-term health and status of bird populations across North America. When combined with other surveys such as the Breeding Bird Survey, it provides a picture of how the continent's bird populations have changed in time and space over the past hundred years.